1. Field of the Invention
This invention relates to an image forming apparatus, such as a digital copying machine, which determines a reference value used for gradation correction from a density histogram of an image, when performing gradation correction on the target image, and carries out the gradation correction using the reference value.
2. Description of the Related Art
Besides conventional analog image forming apparatuses, such as electronic copying machines, digital image forming apparatuses have recently been popularized. In this connection, to realize an automatic exposure function generally found with the analog copying machines, that is, the function of obtaining the optimum picture quality by sensing the manuscript density with a sensor and on the basis of the sensed density, changing the brightness of the exposure lamp, automatic density adjustment using a density histogram has been proposed as described in Jpn. Pat. Appln. KOKOKU Publication No. 64-6588 and Jpn. Pat. Appln. KOKOKU Publication No. 3-30143.
When a reference value for gradation correction is computed from the density histogram of the target image, however, the reference value computed changes, depending on whether the target image is a photograph or character, or with the change of the density distribution in the density histogram. This causes the problem that error correction is unstable in forming an image of a manuscript having photograph areas and character areas intermingled or an image whose density changes greatly.